1. Field of the Invention
The present invention relates to a radial-plunger-type apparatus such as a pump, a compressor, or the like which has cylinders and plungers that extend radially around a main shaft, and more particularly to such a radial-plunger-type apparatus with plungers reciprocally movable in respective cylinders over variable strokes.
2. Description of the Prior Art
One variable-stroke radial-plunger-type apparatus is disclosed in Japanese Patent Application No. 3-37465, for example, filed by the assignee of the present application. The disclosed variable-stroke radial-plunger-type apparatus will be described below with reference to FIG. 16 of the accompanying drawings.
As shown in FIG. 16, the variable-stroke radial-plunger-type apparatus comprises a main shaft (not shown) having an axis C1 and including an integral crankpin 501 having an axis C2 displaced off the axis C1. When the main shaft rotates about its own axis C1, the crankpin 501 revolves around the axis C1. An eccentric collar 502 is relatively rotatably mounted on the crankpin 501. The eccentric collar 502 has an outer circumferential surface 502 a whose center C3 of curvature is displaced off the axis C2 of the crankpin 501.
The variable-stroke radial-plunger-type apparatus has a stroke adjusting mechanism for adjusting the angle through which the eccentric collar 502 is angularly moved on the crankpin 501, i.e., the angular displacement of the eccentric collar 502 on the crankpin 501. The eccentric collar 502 can be held at a certain angle with respect to the crankpin 501, so that the eccentric collar 502 and the crankpin 501 can revolve together around the axis C1 in response to rotation of the main shaft. When the angular displacement of the eccentric collar 502 on the crankpin 501 is adjusted, the distance of the center C3 from the axis C1, i.e., the radius of revolution of the center C3 around the axis C1, is adjusted.
A coupling ring 503 is relatively rotatably mounted on the outer circumferential surface 502 a of the eccentric collar 502 through a bearing 502. To the coupling ring 503, there are connected a plurality of radial rockable plungers 505 through respective coupling pins 504 at circumferentially equal intervals and a single radial fixed plunger 506 integral with the coupling ring 503. The apparatus also includes a cylinder casing 510 rotatable with respect to the main shaft. A plurality of cylinders 508 are rockably mounted on the cylinder casing 510 through respective supporting pins 511 at circumferentially equal intervals around the axis C1. The plungers 505, 506 are slidably fitted in radially inwardly opening cylinder bores in the respective cylinders 508.
When the cylinder casing 510, for example, is held against rotation, and the main shaft is rotated to cause the crankpin 501 and the eccentric collar 502 to revolve around the axis C1, the coupling ring 503 also revolves with the crankpin 501 and the eccentric collar 502 around the axis C1, causing the plungers 505, 506 to reciprocally move in the respective cylinders 508 for thereby drawing a working fluid such as air, oil, etc. into and discharging the working fluid out of the cylinders 508. Thus, the apparatus operates as a pump or compressor.
The rate at which the working fluid is discharged from the apparatus is proportional to the reciprocating stroke of the plungers 505, 506 in the cylinders 508 provided the main shaft rotates at a constant speed. Stated otherwise, the rate at which the working fluid is discharged from the apparatus can be varied or adjusted when the reciprocating stroke of the plungers 505, 506 is adjusted.
The reciprocating stroke of the plungers 505, 506 can be adjusted by the stroke adjusting mechanism which adjusts the angular displacement of the eccentric collar 502 with respect to the crankpin 501. Adjustment of the angle of the angular displacement of the eccentric collar 502 results in adjustment of the radius of revolution of the center C3 around the axis C1. Since the coupling ring 503 is mounted on the outer circumferential surface 502a of the eccentric collar 502, the reciprocating stroke of the plungers 505, 506 is adjusted when the radius of revolution of the coupling ring 503 around the axis C1 is adjusted by the stroke adjusting mechanism.
In the above variable-stroke radial-plunger-type apparatus, when the plunger stroke is adjusted by the stroke adjusting mechanism, the compression ratio of the plunger-piston assembly is simultaneously caused to vary. The compression ratio varies such that the compression ratio is maximum when the plunger stroke is maximum, and the compression ratio is minimum when the plunger stroke is minimum. Therefore, the compression ratio varies according to characteristics which are different from those which are actually required for the apparatus operating as a pump or compressor.
Such compression ratio varying characteristics will be described below with reference to FIGS. 17A, 17B and 18A, 18B of the accompanying drawings.
In FIGS. 17A and 17B, the angular displacement of the eccentric collar 502 is adjusted by the stroke adjusting mechanism to keep the axis C1, the axis C2, and the center C3 (which is also the center of the coupling ring 503) successively in line with each other. At this time, the radius of revolution of the center C3 is maximum, and so is the reciprocating stroke S of the plunger 505.
In the position shown in FIG. 17A, the plunger 505 is positioned at the top dead center. When the crankpin 501 and the eccentric collar 502 turns 180.degree. about the axis C1, they reach the position shown in FIG. 17B, with the plunger 505 positioned at the bottom dead center. At this time, the reciprocating stroke of the plunger 505 from the top dead center to the bottom dead center is indicated by S1. The compression ratio RC1 of the plunger-piston assembly is now represented as follows: EQU RC1=(S1+D1)/D1=S1/D1+1
where D1 is the clearance in the cylinder 108 when the plunger 505 is at the top dead center, i.e., the distance between the upper end of the plunger 505 and the upper end of the cylinder bore in the cylinder 508 when the plunger 505 is at the top dead center.
Then, the angular displacement of the eccentric collar 502 is adjusted by the stroke adjusting mechanism to position the axis C1, the axis C2, and the center C3 as shown in FIG. 18A. The radius of revolution of the center C3 around the axis C1 is reduced, and so is the reciprocating stroke S of the plunger 505.
The plunger 505 is at the top dead center in the position shown in FIG. 18A. When the crankpin 501 and the eccentric collar 502 turns 180.degree. about the axis C1, they reach the position shown in FIG. 18B, with the plunger 505 positioned at the bottom dead center. At this time, the reciprocating stroke of the plunger 505 from the top dead center to the bottom dead center is indicated by S2, and is smaller than the reciprocating stroke S1 shown in FIGS. 17A and 17B. When the plunger 505 is at the top dead center, a clearance D2 is left in the cylinder 508, the clearance D2 being larger than the clearance D1 shown in FIG. 17A. The compression ratio RC2 of the plunger-piston assembly is now represented as follows: EQU RC2=(S2+D2)/D2=S2/D2+1.
Since S1&gt;S2 and D1&lt;D2, the compression ratio RC1 is larger than the compression ratio RC2. In the variable-stroke radial-plunger-type apparatus, therefore, as shown in FIG. 19 of the accompanying drawings, the compression ratio RC is maximum when the plunger stroke is maximum to maximize the rate at which the working fluid is discharged, and the compression ratio RC is reduced as the plunger stroke is reduced to reduce the rate at which the working fluid is discharged.
Generally, if the working fluid is compressible, e.g., if it is air, the operation efficiency of the apparatus is higher as the compression ratio RC is higher. The efficiency of the variable-stroke radial-plunger-type apparatus is maximum when the rate of discharge of the working fluid is maximum. However, as the rate of discharge of the working fluid is reduced, the efficiency of the apparatus is also reduced.
In the case where the variable-stroke radial-plunger-type apparatus is used as an air compressor, for example, it is preferable to supply a reservoir air tank with a large amount of air even under low pressure when the reservoir tank is almost empty, and to supply the reservoir tank with a small amount of air under high pressure when the reservoir tank stores much air and has an increased inner pressure. The variable-stroke radial-plunger-type apparatus poses one problem here in that since the compression ratio is reduced as the plunger stroke is reduced to reduce the rate of discharge of air, the variable-stroke radial-plunger-type apparatus cannot supply air under high pressure when the rate of discharge of air is reduced.